How to tilt (rotate in all axis) a video strip in when editing videos?
https://docs.blender.org/manual/de/dev/editors/vse/sequencer/strips/effects/transform.html

It's not possible, the transform strip only makes 2D transforms.

You can render the video overlay as 3d scene:


a new scene in the VSE project (then it's possible to add the scene directly to the VSE) or a new blend file (then you render the overlay video out and import it to the VSE)
open a 3d view
Use the addon "images as planes" and import your video as a mesh. Select "emit" as shading option.
In the film settings enable "transparent"
Set your iPhone image as background
Enable camera view with numpad-0 key
Transform the movie mesh so that it fits the iPhone screen in the background.
Set the animation length to the frames of the movie.


Two ways to get the overlay in the VSE:


If you used one blend file for both VSE and overlay rendering, then you can add the scene to the VSE.
If you created a new blend file, render the movie out and add it the the VSE. Select "Over drop" or "alpha over" as blend mode.